Disk drive performance is typically measured in data rate, the number of bytes per second that a drive can deliver to a computer processing unit (CPU), and seek time, the amount of time between when the CPU requests a file on a disk drive and when the first byte of the file is sent to the CPU. Disk drives are very fast at moving data sequentially, but are quite slow at moving data randomly scattered on a disk. This is due to the typical configuration of a disk drive which has a platter or disk that rotates and a disk arm that moves over the disk to access data in particular locations on the disk.
A typical modern disk is able to move about 700 KB of data in the time that it takes to move the disk arm to an arbitrary location. Technology trends will make this number increase over time. Because most data transfer times are very small compared to 700 KB of data, in practice disk drives or disks spend most of their non-idle time moving their arm. Additionally, as technology improves, disk transfer rates keep increasing, while disk seek and rotation times shrink very slowly. Therefore, write performance is becoming critical and almost all of it is taken by seeking desired locations on the disk drive.